Devices that transmit signals between input and output circuits that are electrically isolated from each other are presently of considerable commercial importance. For many purposes, electromechanical relays or isolation transformers can both provide adequate electrical isolation and transmit signals between the input and output circuits. However, these devices suffer drawbacks, e.g., large physical size, when compared to solid state circuitry.
To overcome these drawbacks, as well as for other purposes, solid state devices, commonly referred to as opto-isolators, have been developed that use optical coupling between the electrical input and output circuits. The essential elements of the devices are a light source located in the input circuit and a photodetector located in the output circuit and optically coupled to the light source. Many presently used opto-isolators have a GaAs light emitting diode as the light source and a Si photodiode as the photodetector. Light is used in this specification to mean electromagnetic radiation in the visible and near infrared regions, i.e., radiation having wavelengths between approximately 0.5 .mu.m and 2.0 .mu.m. For example, GaAs diodes emit in the infrared. Current in the input circuit passing through the light source causes it to emit light, and some of this light is received by the photodetector and causes an electrical current to be generated or controlled in the output circuit. Opto-isolators provide the advantages of complete electrical isolation between the input and output circuits and small size.
The photodetector may take any of several forms. For example, a silicon photodiode or a photosensitive field effect transistor (FET) may be used. Recently, arrays of series connected photodiodes have been used to photovoltaically control nonphotosensitive field effect transistors. Upon illumination by the light source, the photodiode array produces a voltage between the FET gate and another FET electrode which changes the current conduction state of the FET from its initial to its complementary state. Upon cessation of illumination, the input capacitance of the FET discharges, and the FET returns to its initial current conduction state. Members of this class of opto-isolators are often referred to as optically toggled switches and are available in linear and bilateral embodiments as well as unilateral embodiments.
While these switches represent a significant advance in the art and are perfectly adequate for many applications, they sometimes have one undesirable feature. The bilateral embodiments typically have a shunt resistance connected between the gate and source electrodes to permit the input capacitance of the FET to discharge within a reasonable time after the light source is turned off, and the switch to revert to the initial state. However, the shunt resistance also permits a leakage current to flow when the switch is OFF. There are, of course, many components of the total leakage current such as that due to leakage through the FET. However, the dominant components of the leakage current in the OFF state are a result of the presence of the shunt resistance and are inversely proportional to the sum of the shunt resistance and the load resistance. This sum is normally dominated by the shunt resistance, which is typically in the megohm range, and the leakage currents are therefore generally in the microamp range.